noun
tinder; charcloth
Material used to catch a spark and start a fire, such as charred cloth or dried plant fibers. Often used in traditional fire-starting contexts.
火打ち石で火口に火をつける。
I light the tinder with a flint.
火打ち石 is the flint used to strike sparks, while 火口 is the tinder that catches the spark.
Compound of 火 (hi, 'fire') and 口 (kuchi, 'mouth'), referring to the material that 'takes in' the fire. The reading ほくち is a kun'yomi compound.